[R] Question on Chi-square of null model in sem package
Guo Wei-Wei
wwguocn at gmail.com
Mon Sep 4 07:33:55 CEST 2006
Dear all,
I met a problem while doing SEM by sem package. I got a negative
chi-square of null model. Because the theoretical value of chi-square
cannot be negative, I checked the source code of sem.R in sem package
and I found the Chi-square of null model was computed by the following
expression:
result$chisqNull <- (N - 1) * (sum(diag(S %*% diag(1/diag(S)))) +
log(prod(diag(S))))
I think the reason for negative Chi-square is the too small value of
prod(diag(S)) of my data. I'm working on a data.frame named emc.data
from a sample of a 16-item questioinnaire. The variance of items are
> diag(cov(emc.data))
EMC1 EMC2 EMC3 EMC4 EMC5 EMC6 EMC7 EMC8
0.3622224 0.2350041 0.2488009 0.2901653 0.3195399 0.3107343 0.3436622 0.2345912
EMC9 EMC10 EMC11 EMC12 EMC13 EMC14 EMC15 EMC16
0.2621680 0.3230400 0.4039245 0.3803105 0.2773370 0.4348342 0.2757216 0.3405252
The fit indices of RMSEA and GFI are good, so I think the problem
might be solve by another way for computing the Chi-square of null
model. I'm not well trained in maths, so I come for help. Any advise
is appreciated.
Best wishes,
Wei-Wei
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